THE GEOLOGY AND MINERALOGY OF THE BEIYA SKARN GOLD DEPOSIT IN YUNNAN, SOUTHWEST CHINA

The Beiya skarn gold deposit is located in the eastern Tethyan orogenic belt in western Yunnan province, China. It is one of the largest gold deposits in China, with significant amounts of silver and base metals. To the end of 2014, the estimated resources are 125 million tonnes (Mt) of ore, grading 2.42 g/t Au, 0.48 wt % Cu, 25.5 wt % Fe, 38.85 g/t Ag, 1.24 wt % Pb, and 0.53 wt % Zn. Skarn alteration and mineralization are related to shoshonitic quartz monzonite porphyries that were emplaced in Triassic carbonates (Beiya Formation). Re-Os dating on molybdenite from a skarn orebody indicates an ore-forming age of 36.82 +/- 0.48 Ma, which is consistent with previous dating results of the quartz monzonite porphyries. At least two paragenetic stages of skarn minerals and associated sulfides were recognized, with the early stage typified by garnet +/- pyroxene, magnetite, and calcite, and the late stage characterized by epidote, amphibole, chlorite, quartz, and calcite, containing up to 70% sulfides (pyrite, chalcopyrite, and minor pyrrhotite). The early skarn is dominated by anhydrous minerals, which were replaced by hydrous minerals formed during the late stage. The garnet in the Beiya deposit is andradite rich (Ad(36-97)Gr(3-61)), and pyroxene is relatively diopside rich (Di(8-91)Hd(7-89)). This mineral assemblage indicates an oxidized skarn system, similar to other Au-Cu, Fe-bearing skarn deposits around the world. Fluid inclusions from pyroxene indicate precipitation from high-temperature and high- to moderate-salinity fluids (420-530 degrees C, 11.1-43.3 wt % NaCl equiv), which probably results from boiling of a moderately saline magmatic fluid. Cooler (180 degrees-365 degrees C) and moderate- to low-salinity fluids (1.6-16.5 wt % NaCl equiv) were trapped in garnet and quartz and are interpreted to be responsible for gold deposition. Chlorite chemistry indicates ore-forming temperatures between 300 degrees and 340 degrees C, in agreement with fluid inclusion data. It appears that gold was transported as chloride complexes under oxidized conditions and was deposited at temperatures of about 300 degrees C, when transport of chloride complexes as gold carriers was less efficient.